The present invention relates to low-sulfur lubricating oil compositions, and more particularly to those with excellent anti-wear properties and long-drain properties, suitable for internal combustion engines.
Conventionally, lubricating oils particularly for internal combustion engines are blended with additives containing sulfur and phosphorus such as zinc dialkyldithiophosphates that are extremely excellent in anti-wear and anti-oxidation properties, as substantially essential additives. However, it has been strongly demanded to further decrease the sulfur, phosphorous, and ash of the lubricating oil under the necessity of alleviating harmful effects on exhaust-gas purifying catalysts such as ternary catalysts, oxidation catalysts and NOx adsorbers and exhaust-gas after-treatment devices such as diesel particulate filters (DPF), which are equipped in internal combustion engines in conformity with recent environment-related issues.
Examples of low phosphorus or phosphorus free oils reported so far include those described in patent documents 1 to 10 below while examples of low ash oils include those described in patent documents 11 to 14 below. However, if these conventional oils are decreased in or free of zinc dithiophosphate, it is necessary to add a sulfur-containing compound in the oils in order to maintain the anti-wear properties thereof. The low ash oils contain necessarily zinc dithiophosphate. Therefore, only few lubricating oils with low sulfur, phosphorus and ash with excellent anti-wear properties have been found so far.                (Patent Document 1) Japanese Patent Laid-Open Publication No. 62-253691        (Patent Document 2) Japanese Patent Laid-Open Publication No. 1-500912        (Patent Document 3) Japanese Patent Laid-Open Publication No. 6-41568        (Patent Document 4) Japanese Patent Laid-Open Publication No. 63-304095        (Patent Document 5) Japanese Patent Laid-Open Publication No. 63-304096        (Patent Document 6) Japanese Patent Laid-Open Publication No. 52-704        (Patent Document 7) Japanese Patent Laid-Open Publication No. 62-243692        (Patent Document 8) Japanese Patent Laid-Open Publication No. 62-501917        (Patent Document 9) Japanese Patent Laid-Open Publication No. 62-501572        (Patent Document 10) Japanese Patent Laid-Open Publication No. 2000-63862        (Patent Document 11) Japanese Patent Laid-Open Publication No. 8-48989        (Patent Document 12) Japanese Patent Laid-Open Publication No. 8-253782        (Patent Document 13) Japanese Patent Laid-Open Publication No. 9-111275        (Patent Document 14) Japanese Patent Laid-Open Publication No. 2000-256690        
The inventors of the present invention have discovered that a low sulfur lubricating oil composition containing phosphorus compounds such as zinc mono- or di-alkylphosphate, zinc dialkylmonophosphate and phosphoric acid triester was more excellent in low friction properties, high temperature detergency, oxidation stability, and base number retention properties while maintaining anti-wear properties, than those containing solely zinc dithiophosphate and had filed some patent application based on this discovery (Japanese Patent Application Nos. 2002-015351, 2001-315941, 2002-086145, 2002-086146, 2002-086147, 2002-191090, 2002-191091, and 2002-191092). However, in the case of using a salicylate-based detergent whose metal ratio is 5 or less, particularly that whose metal ratio is adjusted to be 3 or less, as a metallic detergent, a composition can be obtained which is extremely excellent in low friction properties, high temperature detergency, oxidation stability, and base number retention properties. However, it has been found out that when the content of a sulfur- and phosphorus-containing anti-wear agent such as zinc dithiophosphate is decreased or a sulfur-free phosphorus-containing anti-wear agent is used, to decrease the sulfur content, the resulting composition would not be able to exhibit sufficient anti-wear properties for the valve train of an internal combustion engine, such as anti-scuffing properties for the rocker arm pads and anti-wear properties for the cams, and these properties are significantly influenced by the difference in the structure of salicylate to be used.